Floating Knee Injury
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Direct answer
A floating knee injury is a fracture of both the femoral shaft and the tibial shaft on the same limb, which isolates the knee between two unstable segments. The Fraser classification grades it by whether each fracture involves the joint surface, and fixation is planned to restore the length and alignment of both segments in a logical order.
What you must remember
- Definition: fractures of both the femoral and tibial shafts on the same limb, leaving the knee isolated between two unstable segments.
- Mechanism: high-energy trauma, usually road traffic crashes or falls from height, often with injuries to the chest, abdomen or pelvis.
- Fraser classification: type I has both shafts broken without joint-surface involvement, while type II subdivides intra-articular involvement.
- Type II subtypes: IIA has an extra-articular femur and an intra-articular tibia, IIB an intra-articular femur and an extra-articular tibia, and IIC intra-articular fractures in both.
- Modified schemes: some later versions add a type III for extensor apparatus or patellar involvement, so the version used should be stated.
- Assessment: check pulses, compartments and skin, because high-energy trauma raises the risk of vascular injury and compartment syndrome.
- Order of fixation: many surgeons stabilise the femur first, usually with a nail, and then fix the tibia, although the sequence depends on the pattern and associated injuries.
A sequence for assessing and fixing the limb
A 26-year-old man is brought in after a motorcycle collision, with a deformed thigh and a shortened, rotated leg. The steps show the order in which the limb is assessed and fixed, so that the knee is not left unstable at any stage.
- Resuscitate and assess the limb. Manage haemorrhage and the airway first, then examine the pulses, compartments and skin, because high-energy trauma often hides vascular and soft tissue injury.
- Image the whole limb. Obtain full-length films of the femur and tibia with the hip, knee and ankle, add CT where intra-articular extension is suspected, and classify the injury by Fraser.
- Stabilise the femur. Many surgeons fix the femoral shaft first, usually with an intramedullary nail in adults, which restores length and gives a stable reference for the knee.
- Reduce the tibia. Reduce and fix the tibial shaft with a nail or a plate, restoring alignment and rotation so that the knee is coherent.
- Address the joint surface. Fix any intra-articular component with screws or plates, restoring the articular surface before the knee is loaded.
- Monitor and rehabilitate. Watch the compartments and wounds, and begin early knee and hip mobilisation once the construct is stable.
Where the exam tries to catch you
Floating knee questions test whether the candidate stabilises both segments and watches for compartment problems.
- Trap: fixing only the more obvious fracture and ignoring the other segment. Correct: both shafts must be stabilised, because the knee cannot be mobilised while either segment is unstable.
- Trap: missing a compartment syndrome in the leg. Correct: the leg is checked serially after high-energy trauma, since compartments can become tense after surgery.
- Trap: assuming the Fraser grade alone dictates the operation. Correct: the grade informs planning, but articular involvement and vascular status decide the sequence.
Frequently asked questions
What is a floating knee injury?
A floating knee injury is a fracture of both the femoral shaft and the tibial shaft on the same limb, which leaves the knee isolated between two unstable segments. It is a high-energy injury that often accompanies other trauma.
What does the Fraser classification of floating knee grade?
The Fraser classification grades a floating knee injury by whether each fracture involves the joint surface. Type I has both shafts broken without articular involvement, while type II subdivides articular involvement into IIA, IIB and IIC.
In which order is a floating knee injury fixed?
Many surgeons stabilise the femur first, usually with an intramedullary nail, and then fix the tibia, although the sequence depends on the fracture pattern and associated injuries. Any intra-articular component is treated during that sequence.
Why is compartment monitoring important in a floating knee injury?
Compartment monitoring is important because high-energy fractures of the femur and tibia raise the risk of compartment syndrome in the leg. Compartment pressures and neurovascular status are checked serially, especially after surgery.